D-Link DES-3326 User Manual page 409

24-port NWay Ethernet Switch User's Guide
Know which links are redundant.
Organize the redundant links and tune the port cost parameter
of STP to force those ports to be in the blocking state.
For each VLAN, know which ports should be blocking in a stable
network. A network diagram that shows each physical loop in
the network and which ports break which loops is extremely
helpful.
Minimize the number of ports in the blocking state.
A single blocking port transitioning to the forwarding state at an
inappropriate time can cause a large part of a network to fail.
Limiting the number of blocked ports help to limit the risk of an
inappropriate transition.
Understanding and Troubleshooting the Spanning Tree Protocol 393